查看更多>>摘要：Oxidation resistance of gamma - TiAl alloy RNT650 exposed to air and steam at 650 degrees C for 1000 h is reported. The samples were prepared either by conventional casting (CM) or electron beam melting (EBM). The materials were analysed using XPS, XRD, SEM and STEM/EDS techniques. Mass gains in both alloys were comparable and relatively small for air oxidation, but for steam treatment the mass increase in the EBM alloy was up to 50% more than in the CD material. Mass gain showed a faster growth of the scale in the steam treated material. The oxidized materials developed thin nano-crystalline scales predominantly of aluminium and titanium oxides arranged in TiO2+Al2O3/TiO2/Al2O3 sublayers. Whiskers were detected sticking above the scale surfaces, with density much higher in steam treated specimens.
查看更多>>摘要：A eutectoid Ti-7Ni (wt.%) alloy was additively manufactured by electron beam powder bed fusion (E-PBF) in this work. The effects of electron beam current and preheating time on the microstructure and mechanical properties of the E-PBF-built Ti-7Ni (wt.%) alloy were investigated. The evolution of the eutectoid microstructure during the in-situ heat treatment of E-PBF was elucidated. The addition of nickel resulted in a microstructure consisting of pre-eutectoid alpha laths and lamellae of submicron alpha and Ti2Ni. The morphology of the Ti2Ni evolved from fine lamellae to discontinued ellipsoids with increasing electron energy density or preheating time. Defects such as pores and lack of fusion were observed in the alloy under insufficient electron energy density. Fine Ti2Ni lamellae and pre-eutectoid alpha laths were obtained in the alloy processed at an electron energy density of 54 J/mm3 and a preheating time of 6 s, contributing to a considerable ultimate tensile strength of 660 MPa and good elongation of 16.8%. In addition, a graded microstructure along the building direction was observed due to the coarsening of Ti2Ni during multiple thermal cycles. It was demonstrated that the microstructure of the eutectoid Ti-7Ni alloy can be controlled by the in-situ heat treatment during the layer-by-layer E-PBF process. The findings in this work provide understanding and guidance for microstructure regulation in other additively manufactured eutectoid titanium alloys.
查看更多>>摘要：The morphological evolution and grain orientations of intermetallic compounds (IMCs) were investigated during the formation of full Cu3Sn joints at two soldering temperatures (310 and 430 degrees C). The mechanical properties of both types of full Cu3Sn joints were then analyzed. The experimental results showed that the morphology of interfacial IMCs varied significantly at two temperatures. At 310 degrees C, scallop-like Cu6Sn5 grains became larger but were fewer in number and the top of the scallops became rough due to the grain ripening effect. With the increase in the soldering temperature to 430 degrees C, Cu3Sn no longer grew uniformly as observed during soldering at 310 degrees C, but exhibited various growth rates. Furthermore, some eta-Cu6Sn5 transformed into eta'-Cu6Sn5 during cooling, and the following orientation groups between eta'-Cu6Sn5 and eta-Cu6Sn5, i.e., {102}eta' and {1120}eta, {112}eta' and {1010}eta, and {201}eta' and {0001}eta, were found to be near-parallel. The Cu3Sn grains were preferably oriented with [203] and [100] crystal orientations parallel to transverse direction (TD) at both temperatures. Noteworthy, the Cu3Sn grains with [203]//TD and [010]//ND grew much faster than those with other orientations at 430 degrees C. Moreover, molecular dynamics simulation showed that the diffusion coefficients of atoms in Cu3Sn crystals along [203] direction were significantly greater than those in other low-index crystal directions. After soldering, two types of full Cu3Sn joints obtained at different temperatures exhibited different shear strength and fracture mechanisms.
查看更多>>摘要：Developing high-strength and ductile face-centered cubic (fcc) high-entropy alloys (HEAs) has attracted significant attention. The generalized stacking fault energy (GSFE) is a very useful concept to describe stable and unstable planar defects and their energies on a slip plane. It plays an essential role in designing high performance fcc HEAs and understanding the nanoscale plasticity phenomena. In this work, using first-principles simulations, we investigate the configuration-averaged GSFEs of 29 single-phase fcc HEAs and identify indicators that can be used to tune stacking fault energies. First we determine the equilibrium structural parameters for all considered alloys and compare them with available experimental data. With the obtained GSFEs, we analyze the relationship between the stacking fault energies and materials properties, and investigate scaling relations between planar fault energies and the tendencies to exhibit deformation twinning and transformation to hexagonal close-packed martensite. We find that unstable SFE and shear modulus correlates strongly. Moreover, we reveal that the ratio of intrinsic SFE to unstable SFE, gamma isf/gamma usf, is a characteristic materials measure, and the tendencies to twinning and martensitic transformation rank with it. Our results are expected to be useful for an efficient alloy design and selection of solutes in fcc HEAs.
查看更多>>摘要：The quality of metal parts fabricated by selective laser melting (SLM) is strongly depended on the manufacturing process parameters. The establishment of relationship between manufacturing parameters and the formability is the effective way to obtain forming parts with high density, defect-free and excellent mechanical properties by SLM. In this study, the effect of SLM process parameters on the relative density (RD) and mechanical properties of FeCoCrNi high entropy alloys (HEAs) were systematically investigated, involving single tracks, single layers and block samples. By optimizing the laser power, scanning speed and hatching space, the highest RD of 99.85% was obtained under the moderate energy density of 95.24 J/mm3, with no defect as prosity, cracks, un-melted powders and spheroidization under other SLM parameters. The tiny changes in RD significantly affected the mechanical properties of FeCoCrNi HEAs. The highest RD sample corresponded to the optimal comprehensive mechanical properties of 585 MPa in yield strength, 714 MPa in ultimate strength and 45.30% in elongation. The elevation of mechanical properties attributed to the high RD, defect-free, grain refinement and the unique cellular substructure.
查看更多>>摘要：The effect of metallic Nd on the decomposition behavior of Nd6(Fe,Co)13Ga phase in hot-deformed Nd-Fe-B magnet was investigated. The block-shaped Nd6(Fe,Co)13Ga phase in powder flake interfaces of hot-deformed magnet almost decomposed when the metallic Nd diffusion process was performed at 675 degrees C for 30 min. Detailed STEM and corresponding EDS analyses found that three different types of intergranular phases, Nd5Ga3, Nd3Co and nano-sized Nd6Fe13Ga, existed in the Nd-diffused magnet, which indicated a new decomposition mechanism for the Nd6(Fe,Co)13Ga phase. A coupling reaction between the metallic Nd and Nd6(Fe,Co)13Ga phases was designed to confirm the experimental results in the hot-deformed sample after Nd diffusion.
查看更多>>摘要：The evolution of microstructure and recrystallization texture was investigated in a severely deformed (90% coldrolled) and annealed (FCC + BCC/B2) AlCrFe2Ni2 high entropy alloy (HEA) with an exclusive focus on the FCC phase. Cold-rolling resulted in pronounced bending and folding of the lamellae, progressive alignment of the phases along the RD, large misorientation gradient in the FCC phase, and finally ultrafine structure formation. The FCC phase showed a predominantly brass-type (B) texture with orientations spreading from the G ({011}< 100>) to B ({011}<112>), indicating its low stacking fault energy (SFE). The prior cold-rolling deformation strongly influenced the annealing behavior, including lamellar to microduplex structure, grain boundary character evolution, and faster attainment of equilibrium phase fractions. The 90% cold-rolled and annealed material revealed the presence of a remarkably strong B component, unlike the weak recrystallization texture of typical FCC single-phase low SFE HEAs. The origin of the strong B recrystallization component was attributed to a combined effect of the high misorientation gradient in the FCC phase, increased boundary mobility at high annealing temperatures, and the presence of the BCC/B2 phase inhibiting grain growth and thus preserving the recrystallized B grains. The tensile properties of the HEA were evaluated to highlight the outstanding costperformance advantage of the alloy.
查看更多>>摘要：Despite intense interest in the discovery and design of metallic glasses, the efficient a priori identification of novel glass-formers without the need for time-consuming experimental characterization has remained an unattained goal. To address this, we use geometric alignment and density-based clustering algorithms to quantitatively describe the short-range atomic structure in the simulated liquid state for five known metallic glass-forming systems. We show that each liquid is comprised of a surprisingly small number of geometrically-similar atomic clusters (6-8 characteristic motifs in the systems studied) and that the variance of the population distribution of these clusters in the high temperature liquid is inversely correlated to the experimentally-observed glass-forming ability (GFA) as a function of composition within each system studied. These correlations are observed without consideration of temperature-dependent evolution or longer range atomic arrangements, which are much more time-consuming to evaluate. The relative simplicity and broad applicability of this technique to both good glass-forming systems (Cu-Zr, Ni-Nb, Al-Ni-Zr) and poor glass-forming systems (Al-Sm, Au-Si) suggests that the population of characteristic atomic clusters in the simulated liquid could be used as an efficient, high-throughput screening method for identification of potential glass-forming alloys.
查看更多>>摘要：With the continuous expansion of the applications in complex environments, magnetostrictive materials are required to possess not only high performance but also tunable performance. A promising way to achieve this goal is to form composites by combining two or more materials together. However, high performance magnetostrictive composites are still rare, in particular, in a pure metallic form. In this work, we develop a facile and efficient method by using metallic glass as glue, to fabricate pure metallic magnetostrictive composites with high and tunable performance. The Terfenol-D/metallic glass magnetostrictive composites are obtained by a unique thermoplastic forming method under a low temperature (480 K), small stress (114 MPa) and short time (3 min), which exhibit a relatively large field induced magnetostriction (~650 ppm under a magnetic field of 30 kOe) and compressive strength (~185 MPa). Thus, this work provides a promising strategy to design new magnetostrictive materials as well as other metal matrix composites with particular properties.
查看更多>>摘要：The effect of grain size on martensitic transformation was investigated by transmission electron microscopy in a nanocrystalline Ti-16Nb thin film with a broad range of grain sizes. The results show that the larger nanocrystals are filled with multiple variants with the {111}alpha? type I twinning relationship. As the grain size decreases to about 40 nm, the martensite changes to the single variant morphology, and the shape memory effect is believed to deteriorate significantly then. Finally, the martensitic transformation is totally suppressed and the beta parent remains when the grain size drops below 25 nm.
NSTL
Elsevier